ext4: grab page before starting transaction handle in write_begin()

The grab_cache_page_write_begin() function can potentially sleep for a
long time, since it may need to do memory allocation which can block
if the system is under significant memory pressure, and because it may
be blocked on page writeback.  If it does take a long time to grab the
page, it's better that we not hold an active jbd2 handle.

So grab a handle on the page first, and _then_ start the transaction
handle.

This commit fixes the following long transaction handle hold time:

postmark-2917  [000] ....   196.435786: jbd2_handle_stats: dev 254,32
   tid 570 type 2 line_no 2541 interval 311 sync 0 requested_blocks 1
   dirtied_blocks 0

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/inode.c

@@ -875,32 +875,40 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
 		ret = ext4_try_to_write_inline_data(mapping, inode, pos, len,
 						    flags, pagep);
 		if (ret < 0)
-			goto out;
-		if (ret == 1) {
-			ret = 0;
-			goto out;
-		}
+			return ret;
+		if (ret == 1)
+			return 0;
 	}
 
-retry:
+	/*
+	 * grab_cache_page_write_begin() can take a long time if the
+	 * system is thrashing due to memory pressure, or if the page
+	 * is being written back.  So grab it first before we start
+	 * the transaction handle.  This also allows us to allocate
+	 * the page (if needed) without using GFP_NOFS.
+	 */
+retry_grab:
+	page = grab_cache_page_write_begin(mapping, index, flags);
+	if (!page)
+		return -ENOMEM;
+	unlock_page(page);
+
+retry_journal:
 	handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE, needed_blocks);
 	if (IS_ERR(handle)) {
-		ret = PTR_ERR(handle);
-		goto out;
+		page_cache_release(page);
+		return PTR_ERR(handle);
 	}
 
-	/* We cannot recurse into the filesystem as the transaction is already
-	 * started */
-	flags |= AOP_FLAG_NOFS;
-
-	page = grab_cache_page_write_begin(mapping, index, flags);
-	if (!page) {
+	lock_page(page);
+	if (page->mapping != mapping) {
+		/* The page got truncated from under us */
+		unlock_page(page);
+		page_cache_release(page);
 		ext4_journal_stop(handle);
-		ret = -ENOMEM;
-		goto out;
+		goto retry_grab;
 	}
-
-	*pagep = page;
+	wait_on_page_writeback(page);
 
 	if (ext4_should_dioread_nolock(inode))
 		ret = __block_write_begin(page, pos, len, ext4_get_block_write);
@@ -915,7 +923,6 @@ retry:
 
 	if (ret) {
 		unlock_page(page);
-		page_cache_release(page);
 		/*
 		 * __block_write_begin may have instantiated a few blocks
 		 * outside i_size.  Trim these off again. Don't need
@@ -939,11 +946,14 @@ retry:
 			if (inode->i_nlink)
 				ext4_orphan_del(NULL, inode);
 		}
-	}
 
-	if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
-		goto retry;
-out:
+		if (ret == -ENOSPC &&
+		    ext4_should_retry_alloc(inode->i_sb, &retries))
+			goto retry_journal;
+		page_cache_release(page);
+		return ret;
+	}
+	*pagep = page;
 	return ret;
 }
 
@@ -2458,42 +2468,52 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
 						      pos, len, flags,
 						      pagep, fsdata);
 		if (ret < 0)
-			goto out;
-		if (ret == 1) {
-			ret = 0;
-			goto out;
-		}
+			return ret;
+		if (ret == 1)
+			return 0;
 	}
 
-retry:
+	/*
+	 * grab_cache_page_write_begin() can take a long time if the
+	 * system is thrashing due to memory pressure, or if the page
+	 * is being written back.  So grab it first before we start
+	 * the transaction handle.  This also allows us to allocate
+	 * the page (if needed) without using GFP_NOFS.
+	 */
+retry_grab:
+	page = grab_cache_page_write_begin(mapping, index, flags);
+	if (!page)
+		return -ENOMEM;
+	unlock_page(page);
+
 	/*
 	 * With delayed allocation, we don't log the i_disksize update
 	 * if there is delayed block allocation. But we still need
 	 * to journalling the i_disksize update if writes to the end
 	 * of file which has an already mapped buffer.
 	 */
+retry_journal:
 	handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE, 1);
 	if (IS_ERR(handle)) {
-		ret = PTR_ERR(handle);
-		goto out;
+		page_cache_release(page);
+		return PTR_ERR(handle);
 	}
-	/* We cannot recurse into the filesystem as the transaction is already
-	 * started */
-	flags |= AOP_FLAG_NOFS;
 
-	page = grab_cache_page_write_begin(mapping, index, flags);
-	if (!page) {
+	lock_page(page);
+	if (page->mapping != mapping) {
+		/* The page got truncated from under us */
+		unlock_page(page);
+		page_cache_release(page);
 		ext4_journal_stop(handle);
-		ret = -ENOMEM;
-		goto out;
+		goto retry_grab;
 	}
-	*pagep = page;
+	/* In case writeback began while the page was unlocked */
+	wait_on_page_writeback(page);
 
 	ret = __block_write_begin(page, pos, len, ext4_da_get_block_prep);
 	if (ret < 0) {
 		unlock_page(page);
 		ext4_journal_stop(handle);
-		page_cache_release(page);
 		/*
 		 * block_write_begin may have instantiated a few blocks
 		 * outside i_size.  Trim these off again. Don't need
@@ -2501,11 +2521,16 @@ retry:
 		 */
 		if (pos + len > inode->i_size)
 			ext4_truncate_failed_write(inode);
+
+		if (ret == -ENOSPC &&
+		    ext4_should_retry_alloc(inode->i_sb, &retries))
+			goto retry_journal;
+
+		page_cache_release(page);
+		return ret;
 	}
 
-	if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
-		goto retry;
-out:
+	*pagep = page;
 	return ret;
 }